You see the tidy suburban house, the late-model car in the driveway and the lawn of affluent green, and you say, "All right, what's the problem?" What topical scourge will be visited on the inhabitants of this middle-class TV-movie family? In "Consenting Adult," the problem is this: The only son in the house announces he's gay.

Dad runs to his room and sobs, mom trots the kid off to a psychiatrist, and everyone runs around with a very long face. "Consenting Adult" is the big daring TV movie of the year -- say, the year 1975. Or maybe 1965. Here in 1985, the film, which ABC will show tonight at 9 on Channel 7, seems a bit dated, a bygone that perhaps should remain a bygone.

The way the folks in this fictitious but supposedly typical house carry on, you'd think the son had announced he had AIDS, and that would be a more topical subject for a TV movie right now, albeit a difficult one. AIDS is mentioned once in passing, but the film's theme trauma is the son's emergence from the "closet" and how this affects Mom and Dad.

Undoubtedly there continue to be homes where this is or will be a very agonizing situation. According to ABC publicity, "one out of every four American families will have to face the same crisis." But if it's happening in so many homes, surely there are those with circumstances more intriguing and involving than those concocted by writer John McGreevey. The fault may lie in the original 1975 novel by Laura Z. Hobson that McGreevey had to adapt. There's a bland sterility to the characters and the circumstances. They're not far enough from the vaguely drawn anonymities who inhabit educational classroom films, except in their dogged solemnity.

What one can say for the script is that it is carefully written. So carefully it suggests the work of a committee of computers. If homosexuality is depicted negatively in a network film like this, gay pressure groups will object, but if it's depicted too positively, the Bible Belt goes bananas. When the son loses his virginity with another man, for instance, it's something of a television first (and, not to worry, the consummation is tactfully suggested rather than depicted). But McGreevey and director Gilbert Cates have it occur on a dark and stormy night, with thunder (God in his heaven?) clapping angrily overhead.

Basically the treatment is sympathetic. The son has been made a model of masculinity that no gay activist could much object to; he appears to possess not one of the stereotypes associated with homosexual behavior. He's a star of the college swim team, he has no apparent predilection toward promiscuity, he's a regular guy.

In a letter circulated on behalf of the gay newspaper The Advocate, it is stated that "public perceptions of homosexual relationships will alter" after the airing of the film because it offers "the first chance a mainstream audience will have to understand that a younger person's self-discovery is not a choice but a fact of life. They will find parents are not 'to blame.' "

And then they will presumably worry less about whether listening to Culture Club and Frankie Goes to Hollywood records will "turn" their kids gay.

Barry Tubb, who plays the part of the young man at the turning point (and made a name for himself last year on the short-lived "Bay City Blues"), does his best to humanize a character who is largely a symbol, although he has the whiney habit of wrinkling up his nose when complaining or answering an argument. As the mother, Marlo Thomas proves again what a formidable actress she has become, and dominates the film. In fact, this is really her story, isn't it, except that all this data about "the issue" had to be crammed in? Furtive little Martin Sheen is miscast as the father, supposedly a garrulous, back-slapping car salesman, but looking and acting more like a pet mortician.

For reasons not clear, it is established early on that the father has suffered a stroke and recovered. Because of this, it appears he is unable to perform sexually with his wife. What this has to do with the central questions of the film remains a mystery, though it does provide a way to resolve his animosity toward the son's decision. To some extent, the film is a sermon against the notion of parental infliction. Moms and dads should not try to force their children into the roles they want them to play, the screenplay says. The motif is set up early when Thomas chides her daughter (Talia Balsam) for not dressing more fancily. "Your image of me is not my image of me," the daughter says.

Later, the son echoes this plea: "You've got to let me be what I am, because I can't change." The daughter and her husband are portrayed as models for all the world to emulate when it comes to dealing with a family member who is openly homosexual. They are a pat contrast to the mother and father.

Situations in the film do reflect the current official psychiatric and medical wisdom about homosexuality; the movie is more an informational than a dramatic exercise (gay groups will probably welcome it as a consciousness-raiser, even though it's not a very good film). The mother learns, for instance, that her son did not "become" homosexual because she was a suffocating mom. And her initial attempt to have her son "cured" by a shrink fails; the son explains, "He can't cure me. I'm not sick." In 1973, the American Psychiatric Association removed homosexuality from its list of mental disorders.

A year before that, as it happens, ABC aired a film called "That Certain Summer," which also costarred Martin Sheen, except this time he played the gay companion of a married man who departed his closet much later in life than the son in "Consenting Adult." That film seemed brave, if imperfect, at the time, whereas "Consenting Adult" seems honorable, well intentioned and terrifically unmoving. Neither breakthrough nor outrage, it comes across as a variation on "Ordinary People" that is itself too ordinary to be memorable.